The unwritten rule of tabletop Role Playing Games:
The Game/Dungeon Master has the right to veto anything any player says, he has the right to change any rule or make up his own, he need not explain why he choses to do these things. If players complain the GM may choose any of the following to do to the player; slap, call a dumbass, restrict snackage privileges and/or threaten injury to ingame character(be it through loss of xp, health, items or gold)

Player: Can I make my own species for my familiar
DM: Hells no! I'm gonna have to Rule 0 that one
Player: Bitch
DM: I swear I will cut you off of the mountain dew boy
Player: I'm sorry master.

The Urban Dictionary T-Shirt

The unwritten rule in tabletop role-playing games (such as Dungeons & Dragons) which grants the game master the right to suspend or override the published game rules whenever s/he deems necessary.

Similar to a house rule; however, rule zero may be invoked unilaterally, at any time and does not have to be agreed upon in advance by the players.

When abused, can lead to an inconsistent, confusing, or frustrating experience for the players.

Bob: I climb up the wall and into the wizard's castle, bypassing the defenses. I rolled a 20, so I succeed.
GM: The wall is magically warded against climbing. You fail.
Bob: But a 20 is an automatic success!
GM: Rule 0. You fail.

The Urban Dictionary T-Shirt

The unwritten rule of tabletop Role Playing Games (but can be applied elsewhere). It says:

"Don't be a dick."

It essentially means players should treat each friendly and with respect.

If someone intentionally does something inappropriate, or acts maliciously towards another player or players, he's breaking Rule 0.

Particular behavior breaking the rule may differ according to a game you are playing. For example, some RPGs are designed as competitive games, while others are cooperative. Behaviour common in the first type could be sometimes considered bad (breaking the Rule 0) in the latter.

In most games, killing another player's character without proper reason* qualifies as breaking Rule 0.

* other than "I wanted your loot / I wanted to earn XP by killing you / I did it for the lulz" etc...

The Urban Dictionary T-Shirt

This rule applies to people who are clingy to their significant other, and posts their "true love" on their websites. This rule takes them from "cute" and "tolerable" to "straight up annoying" and should be ignored.